The effect of dissolved silicate on the bulk and surface properties of aged ferrihydrite was investigated. Under aging conditions where ferrihydrite crystallizes to goethite in less than 24 h in the absence of silicate, the solid remains noncrystalline for at least 1-2 weeks in its presence. Some of the dissolved Si associates with the solid, and the point of zero charge (PZC) of the solid decreases from about 8 to about 4 as the ratio Si/Fe in the solid increases from 0 to 0.35. Incorporation of the Si in the solid dramatically increases the solid's binding strength for Cd but has relatively little effect on its strength for Cd but has relatively little effect on its strength for Cu, Co, or Zn. The binding strength for SeO3 decreases with Si content in the solid, correlating closely with the PZC.